The long-standing mystery of where Jimmy Carter really stands on national defense was resolved at least temporarily just before Christmas: He is markedly closer to George McGovern tha Scoop Jackson.

The defense budget revealed Dec. 22 generated little immediated uproar, partly because Congress had left town and partly because the significance of the dicision is not fully appreciated. By this budget President Carter actually has broken his promise of 3 per cent annual defense growth, has removed money for strategic-weapon systems tied to current negotiations with Moscow and has junked naval rebuilding.

Completing his first year in office, which included more hours studing the defense budget than any predecessor, Carter is no longer the neophyte on the campaign stump simulstaneously promising defense spending cuts and stronger defense spending cuts and stronger defense. Hence his budget decisions will be interpreted globally by friend and foe as the President's true mind-set.

Superficially, his decision for the fiscal year beginning Oct. 1 does seem that dramamtic. Faced with choosing between the $125 billion proposed by the Office of Management and Budget and the $130 billion requested by Secretary of Defense Harold Brown, the President picked $126 billion.

But that figure is $16 billion below the Ford administration's projection of $142 billion, in itself whittled down by the OMB from Pentagon proposals. Considering inflation, it is actually a decrease from Carter's first budget request (which was cut by Congress). In real terms, it is smaller than the OMB proposal whose opposition by then-Secretary of Defense James Schlesinger cost him his job in 1975. "This is the time of McGovern," one Pentagon insider told us.

Compared with the current budget passed by Congress, the $126 billion provides less than 2 per cent (adjusted for inflation) growth - below the 3 per cent figure pledged by the President to his NATO allies. Thus, Carter has at least tacitly accepted the spurious OMB formulation that his 3 per cent promise need apply only to funds directly tied to NATO.

Basing his decision largely on an OMB memorandum, the President has slashed away at weapons systems at stake in the strategic arms limitation talks. The development of the proposed MX mobile missile is postponed. All funds are eliminated for the sea-launched cruise missile.

But the most dramatic cuts in the Carter budget affect the President's former branch of service. A 2 per cent cutback in real spending for the Navy follows the line of top-secret Carter-administration papers that would be content with convoy mission for the Navy. "It puts the Navy in a Coast Guard status," says an embittered naval officer.

The budget reduces the Navy's requested 30 new ships to 15, but that tells only part of the story. Of those 15 ships, only 10 are combat vessels. Of those 10, two are submarines and the rest are frigates - essentially convoy ships nuclear cruisers have been eliminated.

Moreover, the budget incorporates a key suggestion made to the Carter campaign in 1976 by the Brookings Institution: Severely reduce training funds. The effect on combat readiness, particularly of air and sea power, is causing consternation at the Pentagon.

Hown hard a fight the Defense Department put up against OMB is questionable. Brown went to see the President Dec. 16 carrying a memorandum described by insiders as "wishy-washy" opposing OMB's position. Carter neither debated his Defense Secretary nor announced his intentions after the axfell. Brown told reporters he was "content" with the budget.

Indeed, there is no high official in this administration who will press hard for defense spending. That leads defense-oriented Democrats in Congress to maintain that the President's budget decision merely means he has not been getting strong arguments in the opposition direction and that this budget does not finally unveil his own outlook.

But all the evidence is to the contrary. Coupled with his earlier scrapping of the B-1 bomber, the approval of a budget that erodes strategic-weapons development and sea power must be viewed by the nation and the world - paritcularly the Kremlin - as revealing Carter's real defense philosophy.